material issue - how to make blurry/emissive material?
To truly get transparent edges you would need to move to volumetrics and vary the density or emission based on the proximity to the surface. This can be achieved using the OSL shader described here and produces the following result :



You can vary the 'depth' by adjusting the Multiply node - or replace that and the Color Ramp with your own calculation.

Note that this method will be way less efficient for rendering since it's using volumetrics - and this is further compounded by using OSL which is not GPU compatible (so you'll have to render with CPU).

Using volumetric scatter/absorption as you would for smoke can produce the following :



Adjusting the Multiply and Power nodes affects the sharpness of the objects. You could also add a noise texture to vary the depth of the fade to make the edges appear more 'fluffy'.